The present invention relates to the radio broadcasting and the radio communication technologies, more particularly to an FDM-CDMA transmission method, an FDM-CDMA receiving method, an FDM-CDMA transmitting device, and an FDM-CDMA receiving device, which can integrate the FDM-method broadcasting and the CDMA-method communication.
In long distance radio broadcasting and mobile communication, call quality is extremely unstable due to the multiple propagation in propagation paths. Recently, in mainstream radio and/or wireless telephones are the main stream, sufficient quality has been obtained by frequency modulation (FM) and/or the amplitude modulation (AM). However, the recent data communication typified by the radio and the Internet suffers from extremely serious problems due to the multiple propagation.
Today, two modulation methods are adopted mainly in order to address fading due to the multiple propagation. One modulation method is a code division multiple access (called “CDMA” below) method. The other modulation method is the orthogonal frequency division multiplex (called “OFDM” below) method.
FIG. 1 is a block diagram showing a configuration of a conventional CDMA communication device. FIG. 1A is a block diagram showing a configuration of a CDMA transmitting device. FIG. 1B is a block diagram showing a configuration of a CDMA receiving device. A modulation circuit 1 of the transmitting device performs orthogonal modulation such as quadrature phase shift keying (QPSK) on input digital data in the baseband frequency band. A transmission timing generating circuit 2 generates a transmission timing clock. A PN coding generating circuit 3 generates a pseudonoise signal (PN code), which is synchronized with the sending timing clock. Here, a sufficiently broad band, that is, a rapid PN signal, is used for the bandwidth of the data modulation. A multiplier 4 multiplies digital data output from the modulation circuit 1 by a PN code so that the digital data undergoes spread modulation. A transmitting portion 5 converts a CDMA signal in the baseband frequency band, which is output from the multiplier 4, to a signal in the radio frequency band for radio transmission.
On the other hand, a receiving portion 6 of the receiving device receives and converts a CDMA signal in the radio frequency band to a signal in the baseband frequency band. A timing error detecting circuit 7 performs relative detection of digital data output from the receiving portion 6 and a PN code output from a PN code generating circuit 9 (relative detection), which is described later. As a result of the relative detection, an error in the receiving timing clock is detected. A receiving timing clock corresponding to the detected error is generated from a receiving timing reproducing circuit 8. A PN code is generated from a PN code generating circuit 9, and is synchronized with the received timing clock. A multiplier 10 multiplies the digital data output from the receiving portion 6 by the PN code in order to perform the inverse spread modulation on the digital data. A demodulating circuit 11 performs the orthogonal demodulation such as QPSK on the data output from the multiplier 10.
As described above, the CDMA method is a method for multiplying a normal data modulation signal by a PN code generated rapidly by an unique code assigned to each subscriber In order to further perform the spread spectrum modulation on the normal data modulation signal for transmission. The multiplexing is performed by assigning different codes to different subscribers. Thus, this type of multiplexing is called code division multiplexing. In the CDMA method, the ratio of the information data rate and the spreading code speed (chip speed) is called a spread ratio or a processing gain. Since all of the modulated sending signals occupy a bandwidth corresponding to the chip speed, the frequency bands overlap with each other.
However, when relative-detection is performed by using a PN code of a signal to be received in the receiving device, the electric power of an intended signal becomes larger than other signals in proportion to the squares of the spread ratio while the electric power of the other interference signals becomes larger in proportion to the spread ratio. Therefore, as a result, the S/N ratio is improved in proportion to the spread ratio. This is the reason why the spread ratio is also called processing gain. The CDMA method uses all of the frequency bands for spread modulation. Thus, even if some of the frequency bands are not transmitted due to the multiplex propagation distortion, signals can be transmitted through another band. Therefore, the CDMA method can be used for mobile communication.
On the other hand, the OFDM method is a method for dividing into narrow band frequency channels where the distribution of delays in multiple propagation paths is small enough to performing communication. When the data rate is as large as that of the radio broadcasting, many narrow band channels must be used. Since different frequency channels are not related to each other, that is, they are orthogonal, it is called orthogonal frequency division multiplex. In the OFDM method, frequency division multiplex (FDM) with many narrow bands can be generated easily by the digital signal processing using fast Fourier transforms.
The reason why the OFDM method is used in the mobile broadcasting is that it permits the proper data transmission by using an error correction code as a whole since the other channels are transmitted properly even when some frequency channels are not transmitted due to the transmission path distortion caused by the multiple propagation. This OFDM method is used in the Japanese and European digital broadcasting systems.
Radio communication has an advantage that it can cover a significant amount of the earth's surface all at once compared to wired communication. For example, satellite communication can provide a communication network to a vast area, substantially equal to ⅓ of the earth surface, all at once, by using a single satellite. Satellite communication is particularly suitable for broadcasting and is currently used commercially as direct satellite broadcasting. Application technologies making good use of the advantages of radio are being applied in the fields of broadcasting and mobile communications. The recent evolution of digital mobile communication has been remarkable, and the data communication for telephones and the Internet, as seen in the “i-mode” system of NTT Docomo, has been widely accepted.
The next generation mobile communication system called “IMT-2000” adopts the CDMA method. On the other hands businesses which provide portable or terminal devices moving fast with high quality digital audio broadcasting (called “DAB” below) through a ground relay network or a satellite are emerging. “DAB” adopts the OFDM method. Moreover, the Japanese and European digital broadcasting systems adopt the OFDM method, as described above.
Currently, the mobile communication terminal is a tool used for many purposes as an Internet terminal, and so on. With the spread of mobile communication, a need for receiving DAB from satellites by using the same mobile communication terminal has arisen. However, since the current communication and broadcasting systems adopt completely different communication methods, the integration of communication and broadcasting is extremely difficult.
Furthermore, since the CDMA receiving device must perform relative detection of many chips in an extremely fast manner in the conventional CDMA method, the synchronization of PN codes requires a significant amount of time for inverse spread modulation at the time of the receipt in particular. In a system the cell switching is performed frequently such as in a mobile communication, the long time required for synchronization could be a big problem.